It is customary to employ a water permeable film as a dielectric coated on opposite surfaces with electrodes whereby variations in capacitance across the electrodes is proportional to the moisture content in the space or air in contact with the film. In the past, the film has been mounted on a solid substrate so as not to be fully exposed to the surrounding air, or at least is exposed only on one side, and it has been found that the substrate upon which the film is mounted has a thermal mass which affects the accuracy of measurement and slows the response time of the sensor. Representative capacitive sensing devices are those set forth and described in prior U.S. Pat. Nos. 3,350,941 to K. W. Misevich e al; 3,168,829 to D. E. Nelson and 4,164,868 to T. S. Suntola.
It is therefore desirable to provide a humidity sensing device in which the dielectric film and capacitive electrodes are so mounted and arranged as to be fully exposed to air on opposite surfaces of the film supporting the electrodes; yet, the dielectric film is sufficiently thin as to greatly reduce the response time and increase the sensitivity of the device for measurement purposes. In this regard, it is important to avoid a thermal time constant and to make possible the measurement of high frequency fluctuations in the diffusion of humidity. In other words, in the atmosphere, the humidity is not homogenous and it is therefore important to be able to measure even slight fluctuations or variations in humidity diffusion with respect to time.